Why do keyframed objects cause fluid baking to slow down so much?
Yes. The liquid system must translate one moving object to the domain matrix, making it process for every frame, and calculate the new differences and forces produced by the push of the object over the liquid. Of course, this requires more time. 
I guess if the obstacle is really complex (more faces), this also requires more time.
If you can use lower resolution, less to time to bake. Use lower resolution for tests and increase the resolution for the final product. Take a look at my answer on your other question.